Field of the Invention
The present invention relates to an ESD protection circuit that protects an MIM capacitor, and an RF switch using the same.
Background Art
FIG. 16 is a diagram illustrating a mobile phone RF front-end block. Recent mobile phone RF front-end blocks are mounted with a plurality of power amplifiers and a plurality of duplexers compatible with WCDMA (Wideband Code Division Multiple Access) (registered trademark)/LTE (Long Term Evolution) so as to cover frequency bands which vary from one service/region to another. There are a growing number of cases such as a multi-band power amplifier module (MB PAM) where a GaAs-HBT (heterojunction bipolar transistor) power amplifier and an RF switch are mounted on one module. A plurality of duplexers with different corresponding transmission bands are provided. For example, band B1 corresponds to a transmission band of 1920 to 1980 MHz, and band B2 corresponds to a transmission band of 1850 to 1910 MHz. A switch SW selects one duplexer. FIG. 17 is a functional block diagram that extracts a chain on one side of the multi-band power amplifier module. An RF signal inputted from an input terminal IN is amplified by a power amplifier PA. The amplified signal is outputted from one of output terminals OUT1 and OUT2 selected by an RF switch SW.
Generally, when a power amplifier and an RF switch are commercialized as a module, each terminal of the module needs to satisfy ESD robustness. In the case of portable products,
ESD robustness of 1 kV to 2 kV is required in HBM (human body model) testing. As for ESD surge during HBM testing, a peak current of 0.67 A for 1 kV or 1.3 A for 2 kV is inputted from a terminal normally at a speed of a microsecond order. The current needs to be released to GND within a time of a microsecond order. In the case where the current cannot be released, a transistor, MIM capacitor or wiring may malfunction.
A capacitor having a relatively large value and having almost no influence on RF characteristics is provided at input/output of the RF switch. Along with miniaturization of modules, there are cases where such a capacitor cannot help but be formed on the RF switch. In this case, an MIM capacitor that can be integrated together with the RF switch is usually used for the capacitor. However, since an inter-layer insulating film thickness of the MIM capacitor between upper and lower electrodes cannot be increased so much, a reduction of withstand voltage is unavoidable, and the MIM capacitor often breaks down when a surge current passes therethrough. Therefore, an ESD protection circuit needs to be provided to protect the MIM capacitor. However, there are almost no reported cases on such an ESD protection circuit. As an ESD protection circuit used for the RF switch, a combination of an enhancement-mode (E-mode) HEMT and a depletion-mode (D-mode) HEMT is proposed (e.g., see 2005 IEEE RFIC Symp., pp. 509-512, “An Antenna Switch MMIC for GSM/UMTS Handsets Using E/D Mode JPHEMT Technology”). Moreover, an ESD protection circuit using a D-mode dual gate HEMT is also proposed (e.g., see 2012 IEEE CSIC Symp., “A Novel Electrostatic Discharge (ESD) Protection Circuit in D-Mode pHEMT Technology”).